It is a well-known fact that lithium ion secondary batteries, which have high energy density and whose discharge capacity does not significantly decrease, have been used for a power source for mobile tools such as mobile phones and laptop computers. In recent years, with the miniaturization of mobile tools, there also is a demand for the miniaturization of lithium ion secondary batteries to be mounted therein. In addition, with the development of hybrid cars, solar power generation, and other technologies as a measure to prevent global warming, etc., the application of supercapacitors having high energy density, such as electrical double-layer capacitors, redox capacitors, and lithium ion capacitors, has been increasingly expanding, and there is a demand for a further increase in their energy density.
An electrical storage device, such as the lithium ion secondary battery or the supercapacitor, has a structure in which, for example, a positive electrode, a negative electrode, and a separator made of a polyolefin or the like between them are arranged in an organic electrolytic solution containing a fluorine-containing compound, such as LiPF6 or NR4.BF4 (R is an alkyl group), as an electrolyte. Generally, the positive electrode includes a positive electrode active material, such as LiCoO2 (lithium cobalt oxide) or active carbon, and a positive electrode current collector, while the negative electrode includes a negative electrode active material, such as graphite or active carbon, and a negative electrode current collector, and, with respect to the shape, the electrodes are each obtained by applying the active material to the surface of the current collector and forming the same into a sheet. The electrodes are each subjected to high voltage and also immersed in the highly corrosive organic electrolytic solution that contains a fluorine-containing compound. Accordingly, materials for the positive electrode current collector, in particular, are required to have excellent electrical conductivity together with excellent corrosion resistance. Under such circumstances, currently, aluminum, which is a good electrical conductor and forms a passive film on the surface to offer excellent corrosion resistance, is almost 100% used as the material for a positive electrode current collector. Incidentally, as materials for the negative electrode current collector, copper, nickel, or the like can be mentioned.
In production of electrical storage devices, application of an active material on a surface of a current collector is required to be conducted with a high adhesion, and desirably into a thickness as large as possible in order for the resulting electrical storage device to have a high energy density. When the adhesion between a current collector and an active material is insufficient, the active material is problematically separated from the current collector due to its own volume expansion or the like during the charge-discharge operation. Such problems are more likely to occur as the thickness of the active material applied becomes larger. In particular, LiMn2O4 (lithium manganese oxide), LiFePO4 (lithium iron phosphate) or the like, which has recently attracted attention as new active materials for positive electrodes in place of LiCoO2, generally has a smaller particle size than LiCoO2, and therefore, it is difficult to apply these materials on an aluminum foil to be used as a positive electrode current collector with a high adhesion, unless the surface of the aluminum foil is subjected to any surface treatment or surface processing.
A method of roughening a surface of an aluminum foil through a chemical treatment such as etching is thus proposed in Patent Document 1 as a method for enhancing the adhesion between an aluminum foil and a positive electrode active material. As another method for enhancing the adhesion between an aluminum foil and a positive electrode active material, a method of making an aluminum foil porous by subjecting the foil to a mechanical processing such as punching is proposed.